It is known that the growing of computer applications has imposed the necessity for memories with huge storage capacity needed for libraries, government agencies, hospitals, etc. The new memories should have the following characteristics: low-cost, small size and low energy consumption.
Present memory technologies, such as semiconductor memories, CD-ROMs, rigid and flexible magnetic disks, and magnetic tape store information on a two-dimensional support. Due to their 2-D nature, these memories are not able to provide parallel access, and their access time grows with increasing capacity.
A solution is the use of the third dimension. Three-dimensional optical memories have higher theoretical storage capacity than present 2-D memories.
For example, the maximum theoretical storage density for an optical disk is 1/λ2=3.5×108bits/cm2, while for a 3-D memory 1/λ3=6.5×1012bits/cm3 assuming that the same wavelength of light λ=500 nm is used to access the information. In addition, 3-D optical memory have the potential for parallel access, because an entire plane can be read or written in a single operation. 3-D data storage was experimented on holographic memories made by photorefractive materials (D. Psaltis and F. Mok, Scientific American, November 1995, p.52).